Magnetic recording media comprising a non-magnetic support having provided thereon a magnetic layer containing ferromagnetic particles dispersed in a binder are generally used as audio tapes, video tapes and for computers. Audio tapes and video tapes are encased in a cassette.
Generally, a magnetic layer of such a magnetic recording medium contains carbon black for the purpose of charge prevention. The carbon black generally used has an average primary particle diameter of from 10 to 150 m.mu.. Generally, when carbon black having a small particle diameter is used in a magnetic layer, the surface of the magnetic layer becomes smooth, but the running properties and running durability deteriorate. On the other hand, when carbon black having a large particle diameter is used, the running properties and running durability are excellent, but surface smoothness is poor. Therefore, electromagnetic characteristics, particularly S/N in the case of a video tape, deteriorate.
In light of the above, it was attempted to add carbon black having various average particle diameters in combination into a magnetic layer of a single layer system, as disclosed in JP-B-54-9041, JP-B-53-20203, JP-B-57-12208 and JP-A-58-218039, but a magnetic recording medium having satisfactory both running durability and electromagnetic characteristics could not be obtained. (The term "JP-A" as used herein means an "unexamined published Japanese patent application" and the term "JP-B" as used herein means an "examined Japanese patent publication".)
A multiple layer system, wherein there are at least two magnetic layers, and a method of preparing a magnetic recording medium having both excellent running durability and electromagnetic characteristics is disclosed in JP-A-58-200425. Specifically, JP-A-58-200425 discloses a magnetic layer comprised, an upper layer and a lower layer, of two magnetic layers, wherein only the upper layer contains carbon black to improve running durability and heat resistance. The lower magnetic layer does not contain carbon black, and thus by increasing the packing density of magnetic particles in the lower magnetic layer, the electromagnetic characteristics are improved. However, in the above-described magnetic tape, fine carbon black, i.e. carbon black having an average primary particle diameter of 30 m.mu., is contained only in the upper layer, and the absense of carbon black in the lower layer results in insufficient running durability. Also, the upper layer is made extremely thin layer, i.e., it is made to have a thickness of 1 .mu.m or lower in order to improve the electromagnetic characteristics. However, when the magnetic layer is made so thin, the light-intercepting capability of the magnetic layer decreases. A decrease in light-intercepting capability can cause a VHS system to malfunction by prematurely indicating the end of a tape. This malfunction occure because in the video deck of the VHS system, the device which detects the end of the type is triggered at the point where the tape no longer intercepts light. In other words, when the light-intercepting capability of the magnetic layer is insufficient, the detecting device misinterprets a mid-point in the tape as the end of the tape, which by design has poor light-intercepting capabilities. It is the poor light-intercepting capability of the end of the tape which normally triggers the detecting device. Further, Japanese Patent Application No. 62-136364 (corresponding to U.S. Patent Application Serial No. 07/199,821 filed on May 27, 1988 by some of the coinventors of this invention with others) discloses a magnetic recording medium containing coarse carbon black having a particle diameter of 80 m.mu. or higher in the upper magnetic layer and containing fine carbon black having a particle diameter of less than 40 m.mu. in the lower magnetic layer. Since lower layer of this medium has fine carbon black, the surface smoothness thereof is excellent, and due to the smoothness of the lower layer, the upper layer also has comparatively good surface smoothness. Accordingly, Japanese Patent Application No. 62-136364 provides a magnetic recording medium with excellent electromagnetic characteristics and good running durability.
However, since the upper layer uses coarse carbon black, the dispersibility of carbon black in that layer is not always sufficient, and it is therefore not always possible to obtain both excellent electromagnetic characteristics and good durability. Also, due to the use of coarse carbon black in the upper layer, the light-intercepting capability of the magnetic recording medium tends to be poor, and thus the video decks in VHS systems tend to malfunction because the VHS system's detecting device misinterprets a mid-point in the tape as the tape's end.
Accordingly, a magnetic recording medium having (i) excellent electromagnetic characteristics, (ii) good running durability, and (iii) sufficient light-intercepting capability has not yet been obtained.